Cell phone and credit card.
I usually don't have much extra in the trunk other than lawn chairs, cooler and cleaning supplies when going to car shows.
On a long trip, I would pack an extra 5-gallons of fuel. I have ran out of fuel more often than any other problem.
Jumper cables, and/or one of those Li car jumper packs (small sized, but really powerful.) If you left the lights on, the battery is going bad or alternator belt breaks.
Spare tire, lug wrench, and jack, and any other stuff you might need to change a tire (lock for lugs, hub cap tool, gloves, disposable coveralls?)
For electrical, A leatherman multi-tool, electrical tape and some wire (14 awg, maybe 20' long), maybe an extra fuse or plug in circuit breaker. I don't carry extra bulbs, I can stop at a store to pick them up if needed.
Straps - recovery/tow strap of you go off road, the smaller 1"? ratchet straps. Have used the ratchet strap to hold engine in place when a motor mount broke, also good for holding cargo.
If you have a weak cooling system, pack a few gallons of water.
Safety stuff - first aid kit and road flares. Reminds me I need to replace the road flares, used them at an accident I came up on at night. Almost did not see the vehicles in the road, but was able to avoid them. I pulled over and set out the road flares so on-coming traffic would slow down.
All my cars are electronic ignition with CD ignition or EFI, so if I have an ignition problem that is not really simple, I'm getting towed, but so far ignition system had been very dependable.
For tools, I usually don't pack much for tools, flat head and phillips screwdriver, the leatherman, and maybe 3/8", 7/16", 1/2", and 9/16" combination wrenches. If I am planning on doing mechanical work, like going through a junk yard, I have one of the Craftsman tool sets (I think the 320 piece? that comes in the blow-molded tool box/case. I add a few extra wrenches, wobble extensions, and screwdrivers in the box.